1. Field of the Invention
The invention relates to apparatus and accompanying methods for accurately simulating a digital facility, including impairments, in a public switched telephone network. The invention is particularly suited for precisely emulating a line card within a channel bank including a hardware coder-decoder (CODEC) circuit contained therein, but without using an actual CODEC.
2. Description of the Prior Art
Over the years and particularly with the recent advent of relatively low-cost widespread Internet access, users are increasingly connecting their computers and other computer-based digital devices through a public switched telephone network (PSTN) to, e.g., remote servers of one form or another.
Though local telephone companies (telcos) are expanding their service offerings and modifying their equipment to permit direct digital access, such as through ISDN (integrated service digital network) service, a predominant and still least costly form of telephone service is so-called "plain old telephone service" (POTS). Through this particular service, a local telco provides, what appears to a subscriber, as an analog connection, i.e., a so-called "POTS line", to which that subscriber can simply attach a telephone. Though POTS lines are designed to principally carry human speech, the low cost of POTS service has led to a vast proliferation of devices, such as facsimile machines and computers, being connected, through, either internal or external, modems to such lines.
Apart from an analog connection at a customer site, the PSTN itself is essentially a digital network. Within the PSTN, channel banks are used to convert analog communication on each subscriber loop, e.g., voice (or data) and signaling, to and from a digital form. Each channel bank connects, though a suitable distribution frame and a copper subscriber line, and on a demand basis, a local subscriber, either a caller or called party, to a central office switch. Within the channel bank, each such party is dynamically assigned to and specifically connected to an available one of a number of line cards. A line card, as part of its functions, synthesizes, from incoming digital information supplied by the switch, appropriate analog telephone signaling waveforms, such as for ringing and dial tone, for application to the subscriber line and also provides analog-to-digital (A/D) and digital-to-analog (D/A) conversion for outgoing and incoming voice (or data) communication appearing on or to be applied to that line, respectively. Given that a POTS line is principally intended to carry speech, a filter on each line card limits the signal carried over the POTS line to approximately 4 kHz of bandwidth.
A line card performs the A/D and D/A conversions in hardware through an internal coder-decoder (CODEC) circuit. Such a CODEC contains suitable analog-to-digital pulse code modulation (PCM) and digital (PCM)- to-analog converters and supporting circuitry, and typically is implemented as a single-chip integrated circuit. Since human speech contains most of its energy within a relatively narrow frequency band ranging from above 300 Hz to below 3 kHz, CODECs intended for use with telephony applications have traditionally been designed to optimally operate within this band so as not to appreciably degrade speech. While this band-limited operation adversely affects voice relatively little, if any, it presents difficulties whenever high speed modem communications, such as in excess of 40 Kbaud/second, are to be carried through a POTS line.
In particular, to achieve such high data rates, modem manufacturers are using wider frequency bands, such as approximately 60 Hz (and lower) to approximately 3.7 kHz, than that for which line card CODECs are typically designed to accommodate. Up until rather recently and particularly the appearance of commercially available modems capable of providing data rates in excess of 40 Kbaud/second, there has not been little, if any, practical reason in the art to characterize CODEC performance outside of its traditional 300 Hz-3 kHz band. However, such a reason now exists.
In order to design modems to properly operate over a wide range of line conditions, modem manufacturers often use network simulation equipment, such as that produced by Telecom Analysis Systems of Eatontown, N. J. (which is also the present assignee hereof), to simulate end-to-end performance of a telephone network, particularly various line impairments presented thereby. Through the use of such equipment, modem manufacturers were able to emulate actual line conditions in a laboratory and/or production environment, and observe ensuing modem response and then modify the design of their modems accordingly. Not surprisingly, such conventional equipment included an actual CODEC, as part of its internal circuitry. Not only do line cards contain CODECs, but also so do modems.
Unfortunately, CODEC performance varies among CODECs produced by different manufacturers, as well as even across different lots of the same CODEC that emanate from a common manufacturer. This variability arises at the high-end of the 4 kHz frequency band, typically first appearing above 3.2 kHz, and results from variations in frequency response characteristics of analog filters, typically switched capacitor reconstruction and anti-aliasing filters, contained in the CODEC in both the receive and transmit directions, respectively, at these frequencies. As a result of this variability, high speed modems, particularly so-called "PCM modems" which attempt to attain data rates of up to 56 Kbaud/second, are often not able to uniformly achieve such speeds from one modem to the next even under ideal test conditions. Given this variability in CODECs--which is understood and conventionally accepted by modem manufacturers, such manufacturers often experience some difficulty in guaranteeing high, particularly maximum, speed performance across all their own PCM modems.
Furthermore, conventional network simulation equipment also uses an actual CODEC. Consequently, individual pieces of the same model of such equipment--or differing pieces of equipment, particularly if those pieces were manufactured several years ago and intended for use with relatively low data rate communication, owing to performance variations from one CODEC to the next, can and often do exhibit some variability from one piece to the next when used to simulate network connections for high data rate communication. This, in turn, can inject some variability into the test results taken over time for a common high data rate modem.
Clearly, one skilled in the art can readily appreciate that simulating a PSTN connection, which is to involve a line card that itself exhibits CODEC-induced variability, when using, over time, different pieces of conventional network simulation equipment that also exhibit CODEC-induced variability, greatly complicates the design of high speed, particularly digital, modems. Hence, to ensure uniformity across their network test equipment, manufacturers of such equipment, particularly of models intended for use with high speed data communications, manually test and carefully select a particular CODEC, which exhibits proper high frequency performance, from among those in an incoming lot(s) for inclusion in each piece of equipment then being manufactured. Needless to say, doing so is rather tedious, time-consuming and expensive.
Moreover, at least one apparatus which has been proposed in the art for testing PCM modems would not only rely on using a network simulator that contains a CODEC but also would rely on incorporating a channel bank, and hence another hardware CODEC therein, directly into a data path of a PCM modem under test. In that regard, see "Testing PCM modems", Projects PN-3838 and PN-3857, Contribution to TR-30.3 Subcommittee on PCM Modems, Telecommunications Industry Association, Mar. 4, 1997. Unfortunately, given the inter-CODEC variability, equipment of this sort is rather likely to exhibit undesirable performance variations and hence inconsistencies at high data rates from one installation of this equipment to the next, as well as between this and different network test equipment. This, in turn, would likely inject some error into the test results produced by any of this equipment whenever it is used to test PCM modems.
Clearly, a general need exists in the art for equipment which can accurately simulate actual telephone line conditions through a PSTN that would arise at high data rates, such as would be expected to occur with illustratively PCM modems. Furthermore, a specific need exists to eliminate CODEC-induced variations from such equipment. In that regard, the equipment should accurately emulate the performance of a CODEC throughout an entire frequency range of interest, e.g., approximately 60 Hz (or less) to approximately 4 kHz, needed to support high data rate communication--rather than use an actual CODEC. This equipment should also simulate impairments associated with the connection, including those generated by a line card and a CODEC contained therein. In addition, the equipment should be cost-effective to manufacture. We believe that by using such equipment, modem manufacturers, particularly those of PCM modems, will advantageously be able to more accurately and consistently simulate an actual PSTN connection at high data rates than presently occurs and thus might be able to improve the performance of their high-speed modems.